Move PlatformChannelPair to //mojo/edk/platform and rename it PlatformPipe.

I've temporarily left its tests in //mojo/edk/embedder, since they
depend on platform_channel_utils.*, which I'll move/rename separately.

R=vardhan@google.com

Review URL: https://codereview.chromium.org/1659213002 .
diff --git a/mojo/edk/system/channel_test_base.cc b/mojo/edk/system/channel_test_base.cc
index 67d0a56..91bb997 100644
--- a/mojo/edk/system/channel_test_base.cc
+++ b/mojo/edk/system/channel_test_base.cc
@@ -7,9 +7,10 @@
 #include <utility>
 
 #include "base/logging.h"
-#include "mojo/edk/embedder/platform_channel_pair.h"
+#include "mojo/edk/platform/platform_pipe.h"
 #include "mojo/edk/system/raw_channel.h"
 
+using mojo::platform::PlatformPipe;
 using mojo::util::MakeRefCounted;
 
 namespace mojo {
@@ -63,7 +64,7 @@
 void ChannelTestBase::SetUpOnIOThread() {
   CHECK(io_thread()->IsCurrentAndRunning());
 
-  embedder::PlatformChannelPair channel_pair;
+  PlatformPipe channel_pair;
   raw_channels_[0] = RawChannel::Create(channel_pair.handle0.Pass());
   raw_channels_[1] = RawChannel::Create(channel_pair.handle1.Pass());
 }